Parathyroid glands (PTGs) express a calcium-sensing receptor (CaR) that detects the concentration of extracellular calcium and signals the glands to secrete the proper amount of parathyroid hormone (PTH) to maintain normal calcium levels. The calcium-PTH relationship is sigmoidal and can be defined by four parameters: maximum PTH, minimum PTH, the midpoint of the curve (set-point), and the slope at the midpoint. The CaR levels are decreased in the hyperplastic PTGs of patients with secondary HPT (2oHPT) due to chronic renal failure (CRF), but the factors responsible are not known. The impact of this down-regulation on calcium sensitivity remains controversial. In vitro studies indicate decreased calcium sensitivity (elevated set-point) in parathyroid cells from CRF patients, but in vivo assessment of the set-point in CRF patients have yielded mixed results. Furthermore, in patients it is not possible to relate CaR levels to the four parameters of the calcium-PTH relationship. We have now observed a similar reduced CaR expression in hyperplastic PTGs of CRF rats. This model can be used to determine the role of decreased CaR in PTG hyperplasia and the abnormal calcium-PTH relationship, and to identify the factors that regulate the CaR. Definitive identification of the factors that directly control CaR levels requires an in vitro model with stable expression of the CaR. Traditional monolayer cultures do not respond to calcium. We have developed a unique parathyroid cell culture system that preserves the normal cellular architecture, allowing stable CaR expression and calcium response for several weeks. These cultures, which we refer to as pseudoglands or psGs, provide the best available model for studying the regulation off parathyroid cell CaR expression and provide a unique in vitro model t9o examine the effects in CaR levels on the calcium-PTH relationship. This grant presents an experimental approach that utilizes these in vivo and in vitro models to examine both the causes and effects of the down- regulation of the CaR in renal failure. The specific aims are: 1. To define the relationships, temporal and spatial, between parathyroid gland hyperplasia and down-regulation of the CaR in vivo. 2. To examine the effects of established therapies for uremic secondary hyperparathyroidism (phosphate restriction and vitamin D compounds) on CaR expression in uremic rats. 3. To determine the direct effects of potential regulators of CaR expression in vitro using the psG model. 4. To determine the effects of down-regulation of CaR on the calcium- PTH relationship and PTH expression in vivo and in vitro. These studies will provide new information about the effect of decreased CaR in CRF on the calcium-PTH relationship, define the factors responsible for the decrease, and test therapeutic strategies to correct the abnormality.